1. Field of the Invention
Embodiments of the present invention relate generally to a charged particle beam writing apparatus and a method for detecting irregularities in dose of charged particle beam. More specifically, for example, the embodiments relate to an apparatus and method that detects irregularities in the amount of dose of charged particle beam.
2. Description of Related Art
In recent years, with high integration of LSI, the line width (critical dimension) required for circuits of semiconductor devices is becoming progressively narrower. As a method for forming an exposure mask (also called a reticle) used to form circuit patterns in these semiconductor devices, the electron beam (EB) writing technique having excellent resolution is employed.
FIG. 10 is a conceptual diagram explaining operations of a conventional variable shaped electron beam writing or “drawing” apparatus. The variable shaped electron beam writing apparatus operates as described below. A first aperture plate 410 has a quadrangular aperture 411 for shaping an electron beam 330. A second aperture plate 420 has a variable shape aperture 421 for shaping the electron beam 330 having passed through the aperture 411 of the first aperture plate 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the aperture 411 is deflected by a deflector to pass through a part of the variable shape aperture 421 of the second aperture plate 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g., the x direction) during writing. In other words, a quadrangular shape that can pass through both the aperture 411 of the first aperture plate 410 and the variable shape aperture 421 of the second aperture plate 420 is used for pattern writing in a writing region of the target object 340 on the stage continuously moving in the x direction. This method of forming a given shape by letting beams pass through both the aperture 411 of the first aperture plate 410 and the variable shape aperture 421 of the second aperture plate 420 is referred to as a variable shaped beam (VSB) system.
With the increase in accuracy of an electron beam writing apparatus, demand for improvement in accuracy of detecting writing irregularities is also increased. Therefore, it has been requested to detect even a change of an extremely small amount of dose. Then, in order to detect dose irregularities, it is desirable to directly measure an electron beam during the writing. However, naturally, the electron beam concerned to be measured is being used for writing processing. Therefore, it is difficult to directly measure an electron beam being used for writing.
For example, there is disclosed a method for detecting an irregularity in an electron beam immediately after emitted from an electron gun according to which irregularities are detected by measuring a current amount of beam irradiating a limiting aperture member arranged just below the electron gun (refer to, e.g., Japanese Published Unexamined Patent Application (JP-A) No. 2012-114127).
However, in the variable shaped electron beam (VSB) writing, since a beam shot is sequentially formed while the beam “on” and beam “off” states are alternately repeated, a change occurs in an electron beam by the blanking operation. Accordingly, there is a problem that it is difficult to detect a change of an extremely small amount of dose in such a state.